The following discussion is not an admission that the information described below is common general knowledge among persons skilled in the art.
Powdered activated carbon (PAC) has been used in combination with immersed membranes for drinking water treatment. For example, at the Sioux Lookout Drinking Water Plant in Ontario, Canada, PAC and a coagulant are mixed with feed water. The feed water is stirred in a flocculation chamber to produce floc in the feed water. The feed water is then filtered through an immersed membrane. The PAC dosage rate in a water filtration system is about 25-50 mg/L.
Concerns with the use of PAC in membrane systems include irreversible fouling and abrasion damage to the membranes. The mechanism of irreversible fouling is not fully known. A thesis by Isabel Londono, Assessment of Causes of Irreversible Fouling in Powdered Activated Carbon/Ultrafiltration Membrane (PAC/UF) Systems (The University of British Columbia, 2011), suggests that fouling might be a result of the PAC causing other contaminants in the water to be adsorbed by the membranes. The abrasiveness of PAC is expressed by its Gold Number (GN). PAC selection guidelines for membrane water filtration recommend using PAC brands with a low GN, meaning that they are less abrasive.
A membrane bioreactor (MBR) also uses immersed membranes, but the operating conditions are different than for water filtration. For example, the solids content of mixed liquor in an MBR is much higher than in drinking water and there is no flocculation step. As a result, immersed membranes are air scoured more intensely in an MBR. The concentration of activated carbon that would be required is also much higher, for example 200 mg/L or more. Accordingly, the possibility of PAC fouling or abrading the membranes would be much higher in an MBR than for water filtration.
International Publication Number WO 2009/085252, Suspended Media Granular Activated Carbon Membrane Biological Reactor System and Process, reported that attempting to use powdered activated carbon in an MBR caused significant abrasion to the membranes and non-reversible fouling. A presentation by the inventor (William G. Conner, Oily Wastewater Reuse Technologies, 2011) reported that the abrasion caused up to a 40% reduction in the life expectancy of the membranes. Similarly, US Publication 201202555903 states that attempting to add PAC to an MBR would increase sludge concentration, pore plugging and membrane wear.
To avoid abrasion, WO 2009/085252 describes an MBR in which larger particles of granular activated carbon (GAC) are used. The GAC particles are of a size that can be screened from mixed liquor before the mixed liquor enters the membrane operating system. In this way, the particles are prevented from contact with the membranes.
A membrane bioreactor (MBR) is described in this specification. The MBR has membranes with a supporting structure. The membranes may be, for example, immersed suction driven hollow fiber membranes with a pore size of less than 0.1 microns. A supply unit doses particles of a sorbent, for example powdered activated carbon (PAC), into the MBR. The MBR is configured such that the particles are allowed to contact the membranes directly.
In a process described in this specification, a sorbent such as PAC is dosed into an MBR. The sorbent is maintained at a concentration in the mixed liquor of 200 mg/L or more. Mixed liquor recirculates through in the MBR at a flow rate of at least twice the feed flow rate (2Q). Permeate is withdrawn through immersed membranes comprising a supporting structure. The membranes are operated in a filtration cycle having steps of withdrawing permeate by suction and steps of backwashing or relaxation. Air bubbles are provided to scour the membranes including during at least part of the permeation step. Sorbent particles are present in the mixed liquor and contact the membranes.
In another process, a bioreactor is configured to be able to retain a carrier. One or more bioaugmentation products are added to the reactor. In an example, the one or more bioaugmentation products are screened for their ability to enhance removal of recalcitrant COD. In an example, the one or more bioaugmentation products are immobilized on the carrier before the carrier is added to the reactor. The carrier may be PAC and the reactor may be an MBR.